The present invention relates generally to nuclear reactor fuel assemblies and more particularly to an improved grid for spacing fuel rods and control rod guide thimbles which help constitute a fuel assembly.
Nuclear fuel spacer grids are used to precisely maintain the spacing between fuel rods in a nuclear reactor core, prevent rod vibration, and provide lateral support for the fuel rods. Conventional designs of grids for nuclear reactor fuel assemblies include a multiplicity of interleaved straps of egg crate configuration designed to form cells which accept fuel rods (standard cells) and control rod guide thimbles (thimble cells). Slots are utilized to effect an interlocking engagement with adjacent straps. Each standard cell provides support to one fuel rod at a given axial location through the use of relatively resilient springs and relatively rigid protrusions (called dimples) formed into the metal. A peripheral strap (having the same width as the interleaved straps) encloses the interleaved straps to impart strength and rigidity to the fuel assembly. In order to minimize the lateral displacement of fuel rods and to improve the fuel characteristics of an assembly, a number of grids are spaced along the fuel assembly length. These grids are held in place by attachment to the control rod guide thimbles.
Existing unimetallic grids have springs which are integral with the grid strap and are made from a low neutron capture cross-section material. These springs are subject to stress relaxation under irradiation which could lead to fretting failures of the fuel rods.
Existing bimetallic spacer grids employ grid straps, made from a low neutron capture cross-section material, for structural support and use springs mounted thereto, made from a good radiation stress relaxation material, to contact the fuel rods. A "good radiation stress relaxation material" is one which exhibits low stress relaxation during radiation, meaning the material does not suffer a significant loss of resiliency when exposed to intense and prolonged irradiation. Conventional designs of springs to be attached to a grid strap do not provide for attachment at a location on the grid strap where a fuel rod in a standard cell is on one side of the strap without interfering with a control rod guide thimble in a thimble cell which is directly behind and on the other side of the grid strap.